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In this paper, we propose a joint relay selection and power allocation algorithm for a multi-user, multi relay cellular network using decode-and-forward cooperation strategy with imperfect knowledge of channels. The objective is to minimize the total uplink power of the network taking each user's target data rate as the quality of service constraint under imperfect channel state information (CSI), assuming some statistical knowledge is known about the channel estimation error. Joint relay selection and power allocation is a mixed integer programming problem which is combinatorially hard. For that an efficient sub-optimal solution with low complexity using convex relaxation approach is proposed. After relay selection, a centralized power allocation scheme is developed to optimally allocate the power among the users and the selected relay nodes. Then a low-complexity distributed algorithm is proposed based on a standard primal decomposition approach, where each selected relay can independently minimize its own power. Numerical results demonstrate the effectiveness of the proposed algorithms.